Re: [PATCH v2 1/5] mm/hugetlb: fix races when looking up a CONT-PTE size hugetlb page
From: Baolin Wang
Date: Tue Aug 23 2022 - 08:58:31 EST
On 8/23/2022 4:29 PM, David Hildenbrand wrote:
On 23.08.22 09:50, Baolin Wang wrote:
On some architectures (like ARM64), it can support CONT-PTE/PMD size
hugetlb, which means it can support not only PMD/PUD size hugetlb
(2M and 1G), but also CONT-PTE/PMD size(64K and 32M) if a 4K page size
specified.
So when looking up a CONT-PTE size hugetlb page by follow_page(), it
will use pte_offset_map_lock() to get the pte entry lock for the CONT-PTE
size hugetlb in follow_page_pte(). However this pte entry lock is incorrect
for the CONT-PTE size hugetlb, since we should use huge_pte_lock() to
get the correct lock, which is mm->page_table_lock.
That means the pte entry of the CONT-PTE size hugetlb under current
pte lock is unstable in follow_page_pte(), we can continue to migrate
or poison the pte entry of the CONT-PTE size hugetlb, which can cause
some potential race issues, and following pte_xxx() validation is also
unstable in follow_page_pte(), even though they are under the 'pte lock'.
Moreover we should use huge_ptep_get() to get the pte entry value of
the CONT-PTE size hugetlb, which already takes into account the subpages'
dirty or young bits in case we missed the dirty or young state of the
CONT-PTE size hugetlb.
To fix above issues, introducing a new helper follow_huge_pte() to look
up a CONT-PTE size hugetlb page, which uses huge_pte_lock() to get the
correct pte entry lock to make the pte entry stable, as well as
supporting non-present pte handling.
Signed-off-by: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
---
include/linux/hugetlb.h | 8 ++++++++
mm/gup.c | 11 ++++++++++
mm/hugetlb.c | 53 +++++++++++++++++++++++++++++++++++++++++++++++++
3 files changed, 72 insertions(+)
diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
index 3ec981a..d491138 100644
--- a/include/linux/hugetlb.h
+++ b/include/linux/hugetlb.h
@@ -207,6 +207,8 @@ struct page *follow_huge_addr(struct mm_struct *mm, unsigned long address,
struct page *follow_huge_pd(struct vm_area_struct *vma,
unsigned long address, hugepd_t hpd,
int flags, int pdshift);
+struct page *follow_huge_pte(struct vm_area_struct *vma, unsigned long address,
+ pmd_t *pmd, int flags);
struct page *follow_huge_pmd(struct mm_struct *mm, unsigned long address,
pmd_t *pmd, int flags);
struct page *follow_huge_pud(struct mm_struct *mm, unsigned long address,
@@ -312,6 +314,12 @@ static inline struct page *follow_huge_pd(struct vm_area_struct *vma,
return NULL;
}
+static inline struct page *follow_huge_pte(struct vm_area_struct *vma,
+ unsigned long address, pmd_t *pmd, int flags)
+{
+ return NULL;
+}
+
static inline struct page *follow_huge_pmd(struct mm_struct *mm,
unsigned long address, pmd_t *pmd, int flags)
{
diff --git a/mm/gup.c b/mm/gup.c
index 3b656b7..87a94f5 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -534,6 +534,17 @@ static struct page *follow_page_pte(struct vm_area_struct *vma,
if (unlikely(pmd_bad(*pmd)))
return no_page_table(vma, flags);
+ /*
+ * Considering PTE level hugetlb, like continuous-PTE hugetlb on
+ * ARM64 architecture.
+ */
+ if (is_vm_hugetlb_page(vma)) {
+ page = follow_huge_pte(vma, address, pmd, flags);
+ if (page)
+ return page;
+ return no_page_table(vma, flags);
+ }
+
ptep = pte_offset_map_lock(mm, pmd, address, &ptl);
pte = *ptep;
if (!pte_present(pte)) {
diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 6c00ba1..cf742d1 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -6981,6 +6981,59 @@ struct page * __weak
return NULL;
}
+/* Support looking up a CONT-PTE size hugetlb page. */
+struct page * __weak
+follow_huge_pte(struct vm_area_struct *vma, unsigned long address,
+ pmd_t *pmd, int flags)
+{
+ struct mm_struct *mm = vma->vm_mm;
+ struct hstate *hstate = hstate_vma(vma);
+ unsigned long size = huge_page_size(hstate);
+ struct page *page = NULL;
+ spinlock_t *ptl;
+ pte_t *ptep, pte;
+
+ /*
+ * FOLL_PIN is not supported for follow_page(). Ordinary GUP goes via
+ * follow_hugetlb_page().
+ */
+ if (WARN_ON_ONCE(flags & FOLL_PIN))
+ return NULL;
+
+ ptep = huge_pte_offset(mm, address, size);
+ if (!ptep)
+ return NULL;
+
+retry:
+ ptl = huge_pte_lock(hstate, mm, ptep);
+ pte = huge_ptep_get(ptep);
+ if (pte_present(pte)) {
+ page = pte_page(pte);
+ if (WARN_ON_ONCE(!try_grab_page(page, flags))) {
+ page = NULL;
+ goto out;
+ }
+ } else {
+ if (!(flags & FOLL_MIGRATION)) {
+ page = NULL;
+ goto out;
+ }
+
+ if (is_hugetlb_entry_migration(pte)) {
+ spin_unlock(ptl);
+ __migration_entry_wait_huge(ptep, ptl);
+ goto retry;
+ }
+ /*
+ * hwpoisoned entry is treated as no_page_table in
+ * follow_page_mask().
+ */
+ }
+out:
+ spin_unlock(ptl);
+ return page;
+}
+
struct page * __weak
follow_huge_pmd(struct mm_struct *mm, unsigned long address,
pmd_t *pmd, int flags)
Can someone explain why:
* follow_page() goes via follow_page_mask() for hugetlb
* __get_user_pages() goes via follow_hugetlb_page() and never via
follow_page_mask() for hugetlb?
IOW, why can't we make follow_page_mask() just not handle hugetlb and
route everything via follow_hugetlb_page() -- we primarily only have to
teach it to not trigger faults.
IMHO, these follow_huge_xxx() functions are arch-specified at first and
were moved into the common hugetlb.c by commit 9e5fc74c3025 ("mm:
hugetlb: Copy general hugetlb code from x86 to mm"), and now there are
still some arch-specified follow_huge_xxx() definition, for example:
ia64: follow_huge_addr
powerpc: follow_huge_pd
s390: follow_huge_pud
What I mean is that follow_hugetlb_page() is a common and
not-arch-specified function, is it suitable to change it to be
arch-specified?
And thinking more, can we rename follow_hugetlb_page() as
hugetlb_page_faultin() and simplify it to only handle the page faults of
hugetlb like the faultin_page() for normal page? That means we can make
sure only follow_page_mask() can handle hugetlb.
Mike, Muchun, please correct me if I missed something. Thanks.
What's the reason that this hugetlb code has to be overly complicated?